The “protective member for protecting a back surface of solar cells” as described therein means, in particular, such a member which is used for the purpose of preventing water from penetrating into the solar cells from a back surface side thereof onto which sunlight is not directly irradiated. The protective member is required to have properties such as gas-barrier property, hydrolysis resistance, breakdown resistance, etc. In general, a glass plate has been used as the protective member. However, the glass palate has a less flexibility and causes increase in total weight of the solar cells. Therefore, there is a demand for alternate synthetic resin films which have a light weight and a good flexibility and are capable of satisfying the above various properties as required. Under these circumstances, studies have been made on use of a polyester film as one of these synthetic resin films, which is excellent in mechanical properties, thermal properties, chemical resistance, etc.
On the other hand, as described, for example, in Japanese Utility Model Application Laid-Open (JIKKAI) No. 6-38264, the solar cell module generally has such a structure that a plurality of plate-shaped solar cell elements are intervened between the glass plate disposed on a light-receiving side and a protective membrane disposed on a rear side of the solar cells, and a sealing resin is filled in a clearance inside of the cell. As the sealing resin, a sheet of an ethylene-vinyl acetate copolymer (hereinafter referred to merely as “EVA”) may be frequently used because it is excellent in transparency, moisture resistance and flexibility.
Further, as the sealing resins other than EVA, there are also known polyvinyl butyral (PVB), ethylene-methyl acrylate copolymers (EMA), ethylene-ethyl acrylate copolymers (EEA), ethylene-α-olefin-copolymers or the like.
The polyester films are used as the protective member for protecting a back surface of solar cells by thermally bonding to the above sheet of EVA, etc. However, the polyester films usually have an inert surface, and therefore tend to be deteriorated in adhesion to EVA, etc. For this reason, in order to improve an adhesion property of the polyester films to EVA, etc., there has been proposed the method of providing an easy-bonding layer on a surface of the polyester films (Patent Document 1).
Also, the protective member for protecting a back surface of solar cells has been conventionally employed on the premise that it is used outdoors over a long period of time (for example, over 20 years or longer) and therefore may be exposed to high-temperature and high-humidity environmental conditions. In such a case, as recognized, there tends to occur such a problem that the polyester films suffer from hydrolysis at an ester bond moiety in a molecular chain thereof, so that mechanical properties of the film itself tend to be deteriorated with time. In addition, as another problem, there tends to occur such a fear that an easy-bonding layer provided in the polyester films for improving a thermal bonding property thereof to EVA, etc., is also deteriorated with time under high-temperature and high-humidity environmental conditions, thereby failing to maintain a good adhesion property of the polyester films to EVA, etc.
On the other hand, the solar cells require high initial costs, and an electricity obtained by the solar cells is very expensive as compared to that generated from fossil fuel. Therefore, in order to attain a wide spread of the solar cells, it is necessary to suppress a price of the solar cells. For this reason, it is an important technical background that the protective member for protecting a back surface of the solar cells is formed of a generally-use material as possible to suppress the price of the solar cells.
It is known that the rate of hydrolysis of a polyester becomes higher as a terminal carboxylic acid content in a molecular chain of the polyester is increased. Therefore, there has been proposed such a technique that the carboxylic acid being present in a terminal end of a molecular chain of the polyester is esterified with an epoxy compound to reduce a terminal carboxylic acid content in the polyester and thereby enhance a hydrolysis resistance of the polyester (Patent Document 2). However, the epoxy compound tends to induce gelation of the material upon melt extrusion step in a film formation process or a material recycling process to generate foreign matters, resulting in disadvantages from the viewpoints of environments and costs.
Also, there has been proposed the technique that a terminal carboxylic acid content in the polyester is reduced by adding a carbodiimide such as a polycarbodiimide thereto (Patent Document 3). However, the carbodiimide itself tends to undergo thermal deterioration, so that there is a tendency that the polyester suffers from coloration or deterioration in properties depending upon the reaction conditions used.
In addition, it is known that hydrolysis of the polyester is promoted under acid or alkali environmental conditions (Non-Patent Document 1). Therefore, it is considered that a phosphorus compound such as phosphoric acid and phosphorous acid which is added as a stabilizer for the purpose of preventing undesirable coloration of the polyester upon a polymerization reaction for production thereof gives an adverse influence on hydrolysis resistance thereof because the reaction system becomes acidic upon adding the phosphorus compound thereto.
To solve the problems, there has been proposed the technique that the terminal carboxylic acid content in a polyester is restricted to a predetermined value or lower, and a predetermined amount of a specific phosphoric acid ester is incorporated in the polyester to enhance a hydrolysis resistance thereof (Patent Document 4). However, since the phosphoric acid ester used in the technique has a specific structure, the technique further requires a step of preparing the phosphoric acid ester and costs therefor. Therefore, the technique described in Patent Document 4 is unsuitable for providing a polyester film which can be used outdoors for a long period of time (for example, over 20 years or longer).